Generally, in switching the forward and reverse revolutions of a dc motor, a comparator is used as a means for turning-on or off a driving transistor which is point-symmetrically driven. Or a solenoid is used in order to support a switch knob, and many other devices are proposed. However, all of these devices have problems in driving the dc motor automatically within the upper and lower limits, and in driving the dc motor properly for the respective levels.
Meanwhile, U.S. Pat. No. 4,899,063 which is filed by and granted to the present applicant has the following disadvantage. That is, if the dc motor reaches the upper or lower limit mechanically so as for the dc motor to be no more driven, the point-symmetrically driven transistors Q3-Q6 which are driven by the switchings of transistors Q1 and Q2 are subjected to degradations, or even led to damages. Therefore, there is the disadvantage that the user has to give an attention whenever the switching is terminated.